1. Technical Field
The present invention relates in general to an improved optical system using an optical interference method in the measurement of the flying height of a magnetic head for a magnetic disk and in particular to an optical system wherein, when the interfence light of a magnetic head and a glass disk, which is an alternate to the magnetic disk, undergoes modulation due to a locally uneven internal stress generated in the glass disk during the rapid rotation of the glass disk, the effect of the interference light can be eliminated to obtain a correct flying height.
2. Description of the Related Art
The flying height tester (FHT) which evaluates the flying height of a magnetic head for a magnetic disk storage system is an important apparatus which is essential to the development and manufacture of a magnetic head, and many testers have been developed up to the present. Among them, the FHT of the type which uses the interference of light to measure the flying height is widely used as a tester in production lines because it requires no electrical connection and is noncontacting.
This type of FHT uses a transparent quartz disk or glass disk in place of the magnetic disk and measures the interference color produced by the multiple interference effect of a microscopic gap (on the order of 100 nm between the disk and the magnetic head) to estimate the flying height. This method enables a highly precise measurement, in principle.
FIG. 1 shows a diagram of the FHT. As shown in this figure, light 28 from a light source 25 is reflected at a half-mirror 29 and introduced between the quartz disk 23 and the head 21, such that the reflected light from the disk 23 and the reflected light from the head 21 cause a multiple interference action with each other. This interference light 32 is spectrally divided into at least three wavelength regions and introduced to different light detection means 61, 62, and 63 for the respective wavelength regions. The intensity of the spectrally divided light provided to the respective wavelength regions for the respective detectors depends on the spacing between the disk and the head, or the flying height of the head. Thus, by previously obtaining the relationship between the flying height and the intensity of light for each spectrally divided light, the flying height of the head can be obtained. The principle of the FHT is disclosed in detail in Japanese Patent Application No. 4-297004 filed by this applicant.
FIG. 2 represents the intensity of the reflected light modulated by a multiple interference effect as a function of the wavelength of the light and the flying height. If all of the optical constants of the measuring system, that is, the refractive index and the optical quenching coefficient of the magnetic head surface, are previously obtained for all of the necessary wavelength regions and the spectral sensitivity characteristics of the light source, photodetector, and spectrometer are also known, then a reflection spectrum such as that shown in FIG. 2 is uniquely obtained from the theoretical formulas represented by the following first to fifth expressions. ##EQU1##
In the expressions, n.sub.1, n.sub.2, and n.sub.3 represent the complex index of refraction of the quartz (glass) disk, the air, and the magnetic head, respectively, .lambda. represents the wavelength of light, d represents the air gap length or the flying height, c represents the speed of light, and .omega. represents the angular frequency of the incident light. Further, r.sub.12 represents the reflectivity at the interface between the disk and air, r.sub.23 represents the reflectivity at the interface between air and the magnetic head, and k.sub.i represents the x-component of a wave number vector.
If such functions are prepared in advance, then, by comparing them with the interference colors of the magnetic head flying above the quartz disk the flying height which is not identified can be measured with good accuracy.
In this case, there are two possible methods for determining the flying height. That is, a method for measuring the reflection spectrum of the flying head in a certain wavelength region, seeking out the spectrum related to the same shape as the measured spectrum from the group of spectrums prepared in advance, and obtaining the flying height corresponding thereto, and a method for focusing on the light of several specified wavelengths, and reversely calculating the flying height from the combinations of the intensities of the respective reflected light. The former is called a spectrum evaluation method and the latter is called a three-wavelength method because the measurement is performed using three wavelengths.
The spectrum evaluation method is characterized in that it is easy to determine the flying height because the same spectrum shape is never provided to different flying heights if a proper wavelength range is selected. For this reason, almost all of the FHTs employ this method. However, to derive such a function, it is necessary to determine all of the optical constants of the measuring system, that is, the refractive index and optical quenching coefficients of the magnetic head surface, as well as the spectrum sensitivity characteristics of the light source, photodetector, and spectrometer prior to testing as described earlier. Such measurement requires expensive optical equipment and a high degree of expertise in performing optical measurement. Thus, spectrum evaluation is not appropriate as a calibration method for equipment which is installed in a production line for continuous evaluations.
Furthermore, the small flying height utilized in high-density magnetic disk devices renders the distinction between the spectrum shapes small, making the determination of the flying heights more difficult. In the range of a flying height from 50 nm to 100 nm, there is little change in the spectrum shape in the visible radiation region and only the offset component changes. Since the flying height is estimated from the spectrum shape in this method, it is difficult, in principle, to employ the spectrum evaluation method where the change in the spectrum shape is small.
In the three-wavelength method, the amount of data to be prepared in advance can be substantially smaller compared to the spectrum evaluation method. This method uses three monochromatic lights of different wavelengths, and can uniquely obtain the flying height from the combination of the reflected light intensities giving the same flying height. To have data for measuring the flying height up to 100 nm with a resolution of 1 nm in the spectrum evaluation method, it is necessary to prestore 100 functions, but for the three-wavelength method, it is only necessary to prestore three functions. Thus, in this method, the number of necessary optical constants can be small. However, it is troublesome for an operator to measure the optical constants each time the type of head changes. That is, when the material of the head changes, it is necessary to measure the optical constants using a standard sample with known gap length, but the manufacturing precision of the gap of the standard sample is difficult to maintain as the flying height becomes extremely small. A second problem with the three-wavelength method is that water condensation in the gap can result in an incorrect gap length measurement.
The present inventor proposed a completely new evaluation algorithm different from the conventional one to solve such problems, as disclosed in Japanese Patent Application No. 5-205308. In accordance with this method, a high precision measurement can be performed with a very simple apparatus because an ordinary television camera can be used as a spectrometer while using white light as the light source. That is, in that method, spectral division is performed by the filter of a TV camera, as shown in FIG. 3, and the intensity of the light for a single spectrally divided wavelength range is evaluated in the form of the integrated intensity of light. Then, by specifying the relationships between the combinations of the intensities of light beforehand for the respective spectrally divided wavelength regions and the flying height as shown in FIG. 4, and by comparing them with the measured integrated intensity for each wavelength region, the flying height is obtained.
With reference now to FIG. 5, only the optical system of the FHT is shown. The light generated by the light source 25 passes through the polarizer 503 via the lighting optical system 501. The unpolarized light which has emanated from the light source 25 is converted by the polarizer 503 to a light having only the polarized light for a specific direction, namely, a linearly polarized light. It is now assumed that the light is converted by the polarizer 503 to a light linearly polarized in the direction vertical to the page on which FIG. 5 is drawn. The linearly polarized light is reflected to the glass disk by the beam splitter 505, and passes through the objective lens system containing a zoom mechanism, and the quarter-lambda plate 570 to cause a phase difference of 1/4 of a wave between polarized lights which are oscillating in directions vertical to each other, thereby to convert the linearly polarized light to a circularly polarized light or vice versa. Accordingly, the light linearly polarized when passing through the polarizer is circularly polarized when passing through the quarter-lambda plate, and reflected at the surfaces of the glass disk 23 and the head 21, respectively, to undergo a multiple interference action. These reflected lights are restored to a linearly polarized state when again passing through the quarter-lambda plate. However, the direction of the linear polarization at this time is obtained by rotating the direction of the first linear polarization by 90.degree.. That is, according to the above assumption, the direction of the linearly polarized light which was originally vertical to the page surface is converted to a linearly polarized light having a polarization of a direction horizontal with respect to the page surface by passing through the quarter-lambda plate twice. Then, the light whose polarization direction was rotated by 90.degree. again passes through the objective lens system and reaches the light detection means 515 through the analyzer 513. The polarizer 503 and the analyzer 513 are arranged so that their transmission polarization axes are orthogonal to each other.
The objective of rotating the polarization direction by 90.degree. between the incident light and the detected light is to prevent flare. Flare refers to the light reflected from the inside of the objective lens system (light noise), which is mixed in with the multiple interference light of the reflected light from the glass disk and the head which is to be detected in order to measure the flying height. If the polarization direction is not rotated by 90.degree., then, because the directions of the reflected light (flare) and the multiple interference light are the same, the mixture of these passes through the analyzer to produce a large error. By arranging the polarizer, the quarter-lambda plate, and the analyzer in the above-described form, the polarization direction of the incident light can be rotated by 90.degree. and the multiple interference light and flare can be discriminated from each other by the direction of the linear polarization axis. Further, by making the arrangement such that the transmission axis of the analyzer is coincident with the linear polarization direction of the multiple interference light, only the multiple interference light is allowed to pass, thus preventing the flare from passing.
In such a flare-prevention optical system, the most important element is the quarter-lambda plate for rotating the polarization direction by 90.degree.. By placing the quarter-lambda plate so that its optical axis accurately forms a 45.degree. angle with the polarization direction of the incident light, the polarization direction of incident light can be rotated by 90.degree.. That is, in this optical system, light passes through the quarter-lambda plate twice, in the forward and backward directions, thus causing the same effect as that produced by light passing through a half-lambda plate once, which causes the polarization direction to rotate by 90.degree.. As a consequence, if the angle between the optical axis of the quarter-lambda plate and the polarization direction of the incident light deviates from 45.degree., the polarization direction also deviates from 90.degree.. As a result, the amount of light incident upon the light detection means decreases.
The optical system which prevents flare by rotating the polarization direction by 90.degree. functions perfectly if no factor causing change in the polarization state exists in the optical path. However, the optical system does not function properly if the polarization state of the incident light changes for some reason after the first pass through the quarter-lambda plate and before the second pass. In this case, the incident light is not completely circularly polarized after passing through the quarter-lambda plate the first time and, hence, is not completely restored to a linearly polarized light when passed through the quarter-lambda plate. That is, since a polarization having a component deviating from the transmission polarization axis of the light detector occurs, a change is caused in the apparent intensity of the multiple interference light which should reflect the flying height and thus an accurate flying height cannot be obtained.
FIG. 6 is used to explain this. It is assumed that, for a certain wavelength region, solid line A represents the intensity distribution of a light passing through the analyzer and reaching the light detection means if no factor causing a change in the polarization exists in the optical path. If the polarization state changes, a polarization component deviating from the transmission axis of the light detector is produced, which results in the decrease of the light passing through the analyzer. As a result, the light reaching the light detection means has the intensity shown by broken line B. However, this phenomenon is caused by the change of the polarization state, not by a variation in the intensity of the multiple interference light itself; even if the flying height is the same (the intensity of the multiple interference light is constant), the apparent intensity of light would change. This phenomenon has a remarkably adverse effect on the accuracy of the flying height measurement.
In the FHT optical system, after the incident light passes through the quarter-lambda plate once, the light passes through the glass disk, and again passes through the quarter-lambda plate. The glass disk normally causes no change in the polarization state. However, when an internal stress is applied to the glass disk, birefringence is produced by a photoelastic effect to change the polarization state of the incident light. Such internal stress is produced by, for instance, the residual strain on glass or the unevenness of the tightening force.
Further, internal stress is also generated by centrifugal force due to rotation. In particular, under recent circumstances, where the rotational speed of the glass disk has become very fast, the photoelastic effect due to internal stress by centrifugal force and the change in the polarization state by the development of birefringence due to that effect can has a fatal effect on the accuracy of flying height measurement.
This phenomenon varies according to the relative relationship between the direction of the optical axis of birefringence and the polarization direction of the analyzer in the flare-prevention system. In addition, the change in the amount of reflected light due to the change in the polarization state becomes remarkable, particularly when the quarter-lambda plate is shifted from the position of 45.degree. to perform the overall adjustment of the amount of light (diaphraming).
It is because the quarter-lambda plate is used that the change of the polarization state due to the development of birefringence by a photoelastic phenomenon affects the measurement accuracy. Since the quarter-lambda plate converts only the light having a complete circular polarization to complete linear polarization, it is very sensitive to the change in the polarization state and makes an extremely strong contribution to the change of the intensity of the light detected in the light detector. Thus, the idea of not using the quarter-lambda plate can also be introduced. The reason for this is that the quarter-lambda plate was originally placed to prevent flare, and there is no necessity for using the quarter-lambda plate if flare can be prevented.
One thought is, as shown in FIG. 7, to change the position of the beam splitter from the conventional position between the analyzer and the objective lens system to the position between the objective lens system and the glass disk. If this optical system is employed, no flare can occur since there is no reflection of illuminating light from the inside of the objective lens system. However, this arrangement has the problem that it is difficult to accurately align the optical axis for observing the multiple interference light.
Consequently, it would be desirable to provide an optical system in which the effect of the change in the polarization state involved in the development of birefringence by the photoelastic phenomenon of the glass disk can be reduced as much as possible, while preventing flare and maintaining the position of the beam splitter between the objective lens system and the analyzer as in the prior art. Further, if such an optical system is used, the measurement error can be kept to a minimum even if the glass disk rotates at a higher speed, whereby a high measuring accuracy can be maintained.